Referring to FIG. 1, a block diagram of a conventional refrigeration system 10 employing a central plant architecture, which generally includes a plurality of compressors 12 piped together in an equipment room 6 with a common suction manifold 14 and a discharge header 16 all positioned within a compressor rack 18. The compressor rack 18 compresses refrigerant vapor that is delivered to an outdoor condenser 20 where the refrigerant vapor is liquefied at high pressure. This high-pressure liquid refrigerant is delivered to a plurality of refrigeration cases 22 in a floor space 8 by way of piping 24.
Each refrigeration case 22 is arranged in separate circuits 26 consisting of a plurality of refrigeration cases 22 that operate within a similar temperature range. FIG. 1 illustrates four (4) circuits 26 labeled circuit A, circuit B, circuit C and circuit D. Each circuit 26 is shown consisting of four (4) refrigeration cases 22. Those skilled in the art, however, will recognize that any number of circuits 26 within a refrigeration system 10, as well as any number of refrigeration cases 22 may be employed within a circuit 26. As indicated, each circuit 26 will generally operate within a certain temperature range. For example, circuit A may be for frozen food, circuit B may be for dairy, circuit C may be for meat, etc.
Because the temperature requirement is specific to each circuit 26 but each of the circuits is supplied cooling capacity by a central source, a pressure regulator 28 for each circuit 26 acts to control the evaporator pressure and, hence, the temperature range of the circuit, as dictated by the type of refrigeration cases 22. Typically, each refrigeration case 22 includes an evaporator and expansion valve (not shown), which may be either a mechanical or an electronic valve for controlling the superheat of the refrigerant and thus the temperature of the refrigeration case.
The conventional central plant architecture for a cooling system positions the compressor rack or multiple compressor racks in designated space of a building, perhaps in the equipment room, basement or a rooftop penthouse. In each scenario, the system requires extensive suction and liquid piping throughout the building to feed the refrigeration cases, coolers and/or air conditioning systems. As best illustrated in FIG. 2, liquid and suction piping for each compressor rack A-E must be piped to the associated refrigerated cases 22 in its circuit (as indicated by cross-hatching), often requiring piping to cross the entire building and return. Further, the circuit includes a condenser, which is typically positioned outside the building and requires extensive piping to feed the refrigeration cases, coolers and/or air conditioning systems.
Conventional central plant architecture requires an extensive piping network with suction and liquid piping traversing throughout the store to feed cases, coolers and air conditioning units, which then all run back to a common point, i.e., a suction header for one or more compressor racks. Because of the extensive piping, conventional coding systems require an extensive amount of refrigerant to simply fill the pipes. In addition to the cost of additional refrigerant, the extensive piping network presents a greater opportunity for refrigeration leaks and heat loss, requiring sensors and insulation. Further, the cost and complexity of field piping condensers is significant, as is the physical space required for the central plant or the structural steel to accommodate large central rooftop penthouses.
The communication and power supply network is also extensive as a result of the central plant architecture. With reference to FIG. 1, communication and control wiring for each refrigeration case 22, pressure regulator 28, and sensors 36, 40 are supplied to an analog input board 50 or are received from an input/output board 32 or a driver board 38 to optimize cooling system performance. This extensive network of wires is expensive to design and install. In fact, much of the wiring results from design limitations imposed by the central plant architecture which places the main refrigeration controller 30, input/output module 32, and ESR board 42 in a compressor room 6 and daisy chained via a communication bus 34 to facilitate the exchange of data between them.
For example, to control the various functions of the refrigeration system 10, a main refrigeration controller 30 controls the operation of each pressure regulator 28, as well as the suction pressure set point for the entire compressor rack 18. The refrigeration controller 30 controls the bank of compressors 12 in the compressor rack 18 through the input/output board 32, which includes relay switches to turn the compressors 12 on and off to provide the desired suction pressure. A separate case controller may be used to control the superheat of the refrigerant to each refrigeration case 22 through an electronic expansion valve in each refrigeration case 22 by way of a communication network or bus.
Further, in order to monitor the suction pressure for the compressor rack 18, a pressure transducer 40 may be positioned at the input of the compressor rack 18 or just past the pressure regulators 28. The pressure transducer 40 delivers an analog signal to an analog input board 38, which measures the analog signal and delivers this information to the main refrigeration controller 30, via the communication bus 34. Also, to vary the openings in each pressure regulator 28, the driver board 38 drives up to eight (8) pressure regulators 28. The driver board 38 includes eight (8) drivers capable of driving the pressure regulators 28 via control from the main refrigeration controller 30.
The central plant architecture is particularly inefficient as a result of the compressor rack 18 supplying high-pressure liquid refrigerant to multiple refrigeration circuits operating at different temperatures. With reference again to FIG. 1, the suction pressure at the compressor rack 18 is dependent on the temperature requirement for each circuit 26. For example, assume circuit A operates at 10° F., circuit B operates at 15° F., circuit C operates at 20° F., and circuit D operates at 25° F. The suction pressure at the compressor rack 18, which is sensed through the pressure transducer 40, requires a suction pressure set point based on the lowest temperature requirement for all the circuits 26, which, for this example, is circuit A, or the lead circuit. Therefore, the suction pressure at the compressor rack 18 is set to achieve a 10° F. operating temperature for circuit A, which is able to operate most efficiently with a nearly one hundred percent open pressure regulator 28. Because each circuit 26 is operating at a different temperature, however, the pressure regulators 28 in circuits B, C and D are closed a certain percentage for each circuit 26 to control the corresponding temperature for that particular circuit 26 and costing efficiency. To raise the temperature to 15° F. for circuit B, the stepper regulator valve 28 in circuit B is closed slightly, the valve 28 in circuit C is closed further, and the valve 28 in circuit D is closed even further providing for the various required temperatures. As a result, the central plant architecture dictates certain inherent operative inefficiencies.